(Optionally:
Another way is to install slackpkg+ then run this command to
automatically setup everything that is to be done with the instructions
above:

# /usr/doc/slackpkg+-*/setupmultilib.sh

To uninstall re-run the same command again)

To keep multilib up-to-date:
After that, you can keep multilib up-to-date by running these commands in order, line by line:

# slackpkg update
# slackpkg upgrade-all
# slackpkg install multilib

*To
uninstall multilib, edit the slackpkgplus.conf file located in
/etc/slackpkg/slackpkgplus.conf and remove 'multilib' from PKGS_PRIORITY
and REPOPLUS arrays, but do NOT delete the related MIRRORPLUS line, then run the following commands:

# slackpkg update
# slackpkg remove multilib

Note:
this does NOT remove the core multilib packages, because they affect
the 64bit system. Remove these with the following commands:

# slackpkg upgrade-all

then delete or comment the related MIRRORPLUS line, then run:

# slackpkg update

NOTE: This is a very basic sample slackpkg+ config file could look like with multilib setup:

NOTE: Using this method with slackpkg+ to Install and Keep Multilib
up-to-date in Slackware Current, this is definitely easier than the old
way, and you can do the initial setup manually or with
/usr/doc/slackpkg+-*/setupmultilib.sh script in slackpkg+

Tips and a Fix for Troubleshooting Common Problems with slackpkg/slackpkg+:

-
When errors happen first check the slackpkg+ config file, and the
mirror list at /etc/slackpkg/mirrors and /etc/slackpkg/slackpkgplus.conf
, make sure there are no typos or using of capital letters in the urls
and make sure its all setup as mentioned above.

- Before installing and upgrading with slackpkg/slackpkg+, issue these commands first:

# slackpkg update gpg

then

# slackpkg update

- Sometimes the urls in the mirrors are not up-to-date in this case you have to check and find the right urls that work

-
To avoid problems use urls that are the same version as the system you
use, so use urls with current when using current, or 14.2 with 14.2

-
If you run into an error that says you have duplicate packages that are
conflicting in /var/log/packages, what you have to do is check that
directory of any packages that have either a duplicate of the same
package or

essentially
1 package with double or multiple versions like foo_ver1.1 and
foo_ver1.2 , all you have to do is remove the problematic package in
this case the older one, and it should work.

example issue duplicate slackpkg version conflict:

- When running slackpkg upgrade-all as below you come into contact with such an error:

Checking local integrity... DONE
You have a broken /var/log/packages - with two versions of the same package.
The list of packages duplicated in your machine are shown below, but don't
worry about this list - when you select your action, slackpkg will show a
better list:

slackpkg-2.82.1-noarch-1

You can (B)lacklist, (R)emove, or (I)gnore these packages.
Select your action (B/R/I):

-
Issuing the command removepkg on slackpkg will not fix this issue, even
if you reinstall slackpkg, instead issue this command:

# ls
/var/log/packages | grep slackpkg

-
Fortunately the error gave the hint of the program that is causing
the problem, so you get a list that looks something like this for
example where you see the duplicate issue

slackpkg+-1.7.0-noarch-4mt

slackpkg-2.81.1-noarch-1

slackpkg-2.82.1-noarch-1

-
So you just uninstall the older or problematic version with this
command:

# removepkg /var/log/packages/slackpkg-2.81.1-noarch-1

- Now things should be back to normal, run the # slackpkg upgrade command now and it should work

NOTE: Before installing STEAM and VirtualBox, Wine and PlayonLinux on
Slackware make sure to setup and install multilib in slackware first

-
"https://www.youtube.com/watch?v=AtLM3sjLT7U" *Linux4UnMe Channel
basically mentions he used slackpkg+ to install multilibs in his
slackware linux system, but doesnt explain how he did it, anyways i have tried to explain how to do this in detail above